Can You Use RO Water Purification for Tropical Fish?

All tropical aquarium fish have adapted to the chemical parameters of their home waters. For many species, you need to match these parameters closely, or the fish will get stressed and fail to thrive. The process of reverse osmosis produces water ideal for a number of aquarium fish.

  1. What Is Reverse Osmosis

    • Reverse osmosis -- usually abbreviated RO by aquarium hobbyists -- is a process that uses semipermeable membranes and water pressure to remove dissolved minerals from water. This produces very soft water with almost nothing dissolved in it. Other methods of softening water work great for household applications, but typically produce "soft" water by replacing calcium and bicarbonates with other dissolved minerals. This makes it easier for soap to dissolve in water, and cuts down on hard water stains. But it makes water less like something fish would encounter in the wild and makes it worse for them.

    Soft Water Fish

    • Many tropical fish come from very soft water with few dissolved minerals. For example, most of the Amazon basin is fed by rainwater and snow melt, and the bedrock is chemically inert. This makes the waters of the Amazon very similar to RO water, and species from this region thrive in RO water. Fish from other rain forests, like those of western Africa and Southeast Asia prefer similar conditions. RO water needs minimal preparation to accommodate these fish. However, you should add a small amount of commercial aquarium buffers -- available at pet shops -- to avoid pH swings. Since RO water has no minerals, it doesn't take much to radically alter the pH. Buffers help the water resist pH swings in soft water.

    Salt Mix

    • Salt water is pretty much the opposite of the soft, acidic freshwater of the Amazon. However, tropical marine fish still benefit from RO water. When you keep saltwater fish, you should make your own artificial seawater. Pet shops sell commercial salt mix, which contains salt, buffers and minerals, like calcium and magnesium, that fish and corals need to thrive. However, these mixes are designed to work with RO water, since it is chemically a "blank slate." If you start with water with dissolved minerals in it, you can get salt water with too high a pH. Additionally, RO water is great for tapping off evaporation in a marine tank, since only water evaporates, leaving all the salt and minerals behind.

    Purification

    • Not all tropical freshwater fish prefer soft, acidic water. However, like marine fish, even hard water fish can benefit from RO water. Tap water often contains a number of dissolved compounds, including chlorine, heavy metals and phosphates. Almost universally, these are not at levels that can harm a human. However, fish are much more sensitive to these compounds, so you need to address them. So for tropical fish that like hard, alkaline water, you can use RO water as a starting point, then harden it by mixing in buffers. Similar to salt mix, many pet shops sell cichlid salts which raise the pH and hardness of water. Fish from the African Great Lakes and various parts of Central America benefit from this type of water.